The present invention generally relates to fiber optic materials and methods for producing same. More specifically, this invention relates to fiber optic fabric incorporating woven optical fibers.
Optical fibers have been used in many devices and materials. These fibers may comprise a core and a cladding material. The core may comprise glass or plastic resin. The cladding material may include glass, plastic or air. The cladding material may have a lower refractive index than the core material. The efficiency of light propagation through the fiber may be a function of the difference in refractive index between the core and the cladding.
Light may be introduced at one end of an optical fiber. As the light travels through the fiber, a portion of the light may escape or leak through the sides of the fiber before reaching the end of the fiber. This leakage may be due to light rays exceeding the critical angle for internal reflection. This critical angle may be different for different points along the fiber. Bends in the fiber and scratches on the fiber surface may increase the light leakage at those points.
Using fibers having a core with a substantially higher index of refraction than the cladding may reduce the leakage. The addition of an annular layer of air between the core and the cladding has been described as further reducing the lateral diffusion. By using this type of fiber, lateral diffusion may be minimized.
Some fiber optic applications, however, require light being emitted along the sides of the fiber. The fibers described in U.S. Pat. No. 4,422,719 allow for lateral light emission. These fibers have a clear resinous core and a tightly fitted sleeve. The sleeve is designed to laterally emit a substantial component of light causing the sleeve to have a bright appearance. The entire length of the fiber appears to glow and the fiber is described as useful as a primary light source. Unfortunately, these fibers do not allow for both lit and unlit areas along the fiber. Additionally, they are inefficient at directional lighting.
Others have described ribbons formed by arranging the fibers in a planar, parallel array. One side of the ribbon is roughened by hot stamping with emery paper to form a series of interruptions in the cladding of each fiber. The roughened surface of the ribbon may appear lit when viewed from any angle, while the other side may appear unlit. Unfortunately, it may not be desirable for light to be emitted from the entire surface of the ribbon. Also, the hot stamping may structurally weaken the fibers. Additionally, increased directional lighting control is needed.
Fibers have been bent at selected locations along their length. Although, these fibers may allow for some areas along the fiber to be lit and others unlit, the bending may cause fiber breakage. Additionally, there may be light leakage from the unbent sections of the fiber. Further, this material depends on the transmitted light exceeding the critical angle for internal reflection at the bends. Because light is emitted from these fibers in all directions at the bends, directional control of the light emission is inefficient.
Fibers with luminaires and fibers with notches have been used to emit light in some manner directionally from the side of the fiber. Fibers with luminaires require the termination of a fiber at the luminaire thus creating the need for large numbers of fibers to feed an array of emission points. Also, the use of luminaries may have an adverse effect on aerodynamics, such as when the luminaires are attached to the outer mold line (OML) of an aircraft. Notched fibers may laterally emit light, but the notches may weaken the fibers. The notch creates a stress concentration that may fracture the fiber under minor loading. Some notched fibers have received additional processing. Notched fibers have been reflectively coated on one surface of the notch so that the light is reflected in a direction substantially perpendicular to the fiber. Although these processed notched fibers may allow for some light emission control, further increases in directional control are needed.
Woven and non-woven optical materials comprising these fibers have been described. A light emitting fabric comprising optical fibers woven into a cloth is disclosed in U.S. Pat. No. 4,234,907. Optical fibers may not be totally efficient at transmitting light and some light may leak into the cladding and produce a glow along the length of the fiber. These fabrics may comprise a combination of optical and non-optical fibers and may be uniformly illuminated. In such a fabric, the optical fibers may be visible when the fabric is unlit. Unfortunately, concealment of the optical fibers when unlit may be desired in some applications. Additionally, a fabric having uniform light emission may not be efficient when directional light emission is desired.
As can be seen there is a need for improved fiber optic materials. Specifically, an improved fiber optic fabric is needed, wherein there is increased directional control of the light emission from the fibers. Also needed is a fiber optic fabric having decreased light leakage and increased fiber strength. Further, a fiber optic fabric having increased directional emission control without the use of luminaries is needed. Additionally, a fiber optic fabric is needed, wherein the optical and non-optical fibers appear less distinctive when the fabric is unlit.